This application is based on Patent Application No. 2000-335179 filed Nov. 1, 2000 in Japan, the content of which is incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to an ink jet printing method and apparatus on a multi-path printing system in which a desired image is formed completely on a printing medium by a plurality of scanning movements of a printing head on the same printing area of the printing medium.
2. Description of the Related Art
FIGS. 1A, 1B, and 1C are schematic diagrams that illustrate a transport stage for transporting a printing medium in a typical serial type printing apparatus.
In those figures, the reference numeral 509 denotes a printing medium (e.g., a sheet of paper or a plastic sheet). The printing medium 509 can be fed in a sub-scan direction from the right side to the left side as indicated by the arrow in each figure. In other words, the printing medium 508 receives the sheet-feed force when it is sandwiched between the transport roller 501 and the pinch roller 503. The transport roller 501 can be driven by a sheet-feed motor (not shown). The pinch roller 502 is pressed against that transport roller 501 by the spring pressure, so that it can be rotated together with the transport roller 501. The sheet-ejection rollers 503, 504 are also arranged as a two-stage sheet-ejection roller so that they work together with the transport roller 501. In addition, sheet-ejection spurs 505, 506 are pressed against the rollers 503, 504 by the spring pressures, respectively.
The feed amount of the printing medium 508 (the amount by which, or rate at which, it is fed) due to the rotational movements of the sheet-ejection rollers 503, 504 is adjusted to about 100.3% of the feed amount of the printing medium 508 due to the rotational movement of the transport roller 501 because the printing medium swells due to the application of ink upon printing in an ink jet printing system. If the feed amount of the printing medium 508 defined by the sheet-ejection rollers 503, 504 were set to the same level as that due to the transport roller 501, the swell amount of the printing medium 508 would accumulate, causing a failed sheet-feed between these rollers.
In the sheet-feed stage shown in FIG. 1A, the printing medium 508 becomes caught between the rollers using the upper portions of the sheet-ejection rollers 503, 504, and the transport roller 501, respectively. If the printing medium 508 did not swell under the condition shown in FIG. 1A, the printing medium 508 would slip over the contacted surfaces of the respective sheet-ejection rollers 503, 504. In this case, therefore, the feed amount of the printing medium 508 could depend on the transport thereof by the transport roller 501.
Furthermore, the reference numeral 507 denotes a printing head that performs a printing scan in the direction perpendicular to the plane of each of FIGS. 1A, 1B, and 1C (i.e., main-scan direction). The reference numeral 509 denotes a printing area of the printing medium 508 to be printed by the printing head 507.
FIG. 1A illustrates the state in which two distinctive sheet-feed forces are being applied on the printing medium 508 by the transport roller 501 and sheet-ejection rollers 503, 504, respectively. FIG. 1B illustrates a moment when the rear end of the printing medium 508 has left the space between the pinch roller 502 and the transport roller 501. FIG. 1C illustrates the state in which the sheet-feed force is being applied to the printing medium 508 only by the sheet-ejection rollers 503, 504.
Referring now to FIG. 2, a two-path printing system will be described as an example of the conventional multi-path printing system, in which image formation is completed by moving the printing head 507 through two scans in forming each pixel.
The printing head 507 in FIG. 2 relatively moves towards the bottom of the figure with respect to the printing medium 508, which is intermittently transported in the sub-scan direction. As the present example is the two-path printing system, as those of the steps S201 to S207 (shown as successive head positions in FIG. 2.), the position of the printing head 507 in the sub-scan direction relatively deviates downward by one-half of a printing element width in the sub-scan direction. An image can be formed on each of the printing areas P0 to P14 on the printing medium 508 by two printing scans of the printing head 507. In the figure, xe2x80x9cA2xe2x80x9d denotes a mask that allows 50% of image data to remain. xe2x80x9cB2xe2x80x9d denotes a mask that interpolates the image data that mask A2 excludes.
The images are formed on the printing medium 508 one after another by alternately repeating: the printing scan of the printing head 507 on the basis of image data thinned out by alternately using the masks A2 and B2; and the transport of the printing medium 508 in the sub-scan direction by shifting it by one-half of the printing element width of the printing head 507. More specifically, in the step S201, the printing scan is performed on the basis of the image data thinned out using the mask A2, and thereafter, the printing medium 508 is fed in the sub-scan direction by one-half of the printing element width. Subsequently, in the step S202, the printing scan is performed on the basis of image data thinned out using the mask B2, and thereafter, the printing medium 508 is fed in the sub-scan direction by one-half of the printing element width. Thereafter, the same procedure is successively repeated to form images on the printing medium 508 in succession.
In the multi-path printing system of FIG. 2, for example, the following description relates to a period at step S204, which corresponds to the state shown in FIG. 1A and a period at step S205, which corresponds to the state shown in FIG. 1B. In this case, during the transport of the printing medium 508 from S204 to S205, the rear end of the printing medium 508 moves through a point PA between the steps S204 and S205, where it emerges from the grip of the pinch roller 502 (i.e., comes into the state shown in FIG. 1B).
The printing head 507 may be an ink jet print head having a plurality of printing elements that eject ink from their nozzles. In this example, the number of nozzles is 256 with 1200-dot/inch resolution. In this case, it is possible to form ink dots uniformly on the printing medium 508 as shown in FIG. 3 under the condition at step S204, which corresponds to the state shown in FIG. 1A. In the state of FIG. 1A, the printing medium 508 may be intermittently fed in the sub-scan direction to distances of about 2700 xcexcm, that correspond to 128 nozzles. In the period at S205, which corresponds to the state shown in FIG. 1C, the feed amount of the printing medium 508 may be about 2708 xcexcm, corresponding to about 100.3% of about 2700 xcexcm. As a result, as shown in FIG. 4, each of the positions on which ink dots have been formed deviates by about 8 xcexcm from its ideal position in the sub-scan direction, where that deviation corresponds to the difference between the feed amounts of the printing medium 508 obtained at the states shown in FIG. 1A and FIG. 1C. Furthermore, if the printing medium 508 is shifted from the state of FIG. 1A to the state of FIG. 1C through the state of FIG. 1B, the feed amount of the printing medium 508 may be further increased by about 8 xcexcm in addition to the mentioned difference in feed amount of 8 xcexcm. Because it is conceivable that the difference between the feed amounts of the sheet-ejection rollers 503, 504 and the transport roller 501 cannot be substantially removed by the slippage of the sheet-ejection rollers 503, 504, the stresses stored in the printing medium 508 and the sheet-ejection rollers 503, 504 are released at the instant when the printing medium 508 comes off the pinch roller 502. In other words, when the printing medium 508 is shifted to the state of FIG. 1C through that of FIG. 1B, a distance of about 8 xcexcm, corresponding to the difference between the feed amounts of the sheet-ejection rollers 503, 504 and the transport roller 501 is added to another distance of about 8 xcexcm that results from detachment of the printing medium 508 from the pinch roller 502. As a result, as shown in FIG. 5, the ink-dot-forming position is displaced from the ideal ink-spotting position by up to a maximum of the sum of those two amounts (i.e., about 16 xcexcm in total).
The printing density, i.e., the number of ink dots which can be formed per unit area of the printing medium 508, in each of FIGS. 3, 4, and 5 shows a correlation with the coverage of ink on the printing medium 508, which is given by the following expression: [Printing density in FIG. 3] greater than [Printing density in FIG. 4] greater than [Printing density in FIG. 5].
In the case of FIG. 2, the printing medium 508 comes off the pinch roller 502 during the transition from the step S204 to the step S205. Thus, the printing areas up to P0 in FIG. 2 have the printing density shown in FIG. 3, the printing areas from P5 on have the printing density shown in FIG. 4, and the printing areas from P1 to P4 have the printing density shown in FIG. 5. As a result, uneven density can be observed as different-density bands generated in the resulting image, as shown in FIG. 6.
An object of the present invention is to provide a method and apparatus of printing an image on a printing medium, in which the printing speed is reduced as much as possible, so that the deterioration of the image quality on the latter part of the printing medium in the transport direction can be substantially reduced.
In a first aspect of the present invention, there is provided a printing apparatus for printing an image on a printing medium by repeating a printing scan of a printing head having a plurality of printing elements in a main-scan direction and a transport of the printing medium in a sub-scan direction perpendicular to the main-scan direction, comprising:
first printing control means for providing a complete image by performing printing scan for M times (M is an integral number: Mxe2x89xa71) using N printing elements (N is an integral number: Nxe2x89xa72) on the same printing area of the printing medium;
second printing control means for providing a complete image by performing printing scan for K times (K is an integral number: K greater than M) using L printing elements (L is an integral number: Lxe2x89xa6N) on the same printing area of the printing medium; and
printing-control switching means for allowing an image printing using the first printing control means for each printing area in a front side of the printing medium in the direction of transporting the printing medium and an image printing using the second printing control means for each printing area in a rear side of the printing medium in the direction of transporting the printing medium.
In a second aspect of the present invention, there is provided a printing method for printing an image on a printing medium by repeating a printing scan of a printing head having a plurality of printing elements in a main-scan direction and a transport of the printing medium in a sub-scan direction perpendicular to the main-scan direction, comprising the steps of:
providing a complete image by performing printing scan for M times (M is an integral number: Mxe2x89xa71) using N printing elements (N is an integral number: Nxe2x89xa72) on the same printing area in a front side of the printing medium in the direction of transporting the printing medium; and
providing a complete image by performing printing scan for K times (K is an integral number: K greater than M) using L printing elements (L is an integral number: Lxe2x89xa6N) on the same printing area in a rear side of the printing medium in the direction of transporting the printing medium.
In a third aspect of the present invention, there is provided a printing apparatus for printing an image on a printing medium by repeating a printing scan of a printing head having a plurality of printing elements in a main-scan direction and a transport of the printing medium in a sub-scan direction perpendicular to the main-scan direction, comprising:
first printing control means for providing a complete image by performing printing scan for M times (M is an integral number: Mxe2x89xa71) on the same printing area of the printing medium;
second printing control means for providing a complete image by performing printing scan for K times (K is an integral number: K greater than M) on the same printing area of the printing medium; and
printing-control switching means for initiating an image printing by the first printing control means in a printing operation on the printing medium and switches from the image printing using the first printing control means to an image printing using the second printing control means depending on a transporting position of the printing medium.
In a fourth aspect of the present invention, there is provided a printing method for printing an image on a printing medium by repeating a printing scan of a printing head having a plurality of printing elements in a main-scan direction and a transport of the printing medium in a sub-scan direction perpendicular to the main-scan direction, comprising the steps of:
initiating an image printing on the printing medium by a printing operation in which a complete image is obtained by performing printing scan for predetermined times on the same printing area of the printing medium; and
switching from the printing operation to another printing operation in which a complete image is obtained by performing printing scan for more times than the predetermined times on the same printing area of the printing medium.
According to the above aspects of the present invention, the printing movement of the printing head can be altered so that the number of the printing scan of the printing head for the same printing area on the printing medium can be increased. Consequently, the printing speed is reduced as much as possible, furthermore the deterioration of the image quality on the latter part of the printing medium in the transport direction can be substantially reduced.
The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings.